There is conventionally known a structure of a thermal-type flow measuring device including the following housing and flow sensor. Specifically, in this known structure, the housing includes a sub passage through which measured gas flowing in a main passage is taken in to flow, and the flow sensor is disposed in the sub passage to generate a signal in accordance with the flow rate of the measured gas flowing in the main passage by its heat transfer with the measured gas flowing through the sub passage. The flow measuring device returns the measured gas which has passed through the flow sensor into the main passage through the outlet of the sub passage.
When such a flow measuring device is used for measuring a flow rate of intake air drawn into an internal-combustion engine, the flow measuring device needs to measure not only a flow rate when intake air flows in a forward direction but also a flow rate when intake air flows in a backward direction. Specifically, the flow rate of intake air pulsates based on the operation of the internal-combustion engine, and when the operational state of the internal-combustion engine is in a particular region, the intake air flows in the backward direction as well as in the forward direction due to the pulsation, so that a backflow phenomenon of intake air is periodically repeated.
Thus, to measure the amount of intake air taken into the internal-combustion engine, the flow rate at the time of a backward flow as well as the flow rate at the time of a forward flow may be measured to obtain the amount of intake air taken into the internal-combustion engine based on the flow rates both at the time of a backward flow and at the time of a forward flow. For this reason, there is known a flow measuring device that takes measured gas into the sub passage from the outlet at the time of a backward flow to make the flow sensor sense the gas and that can also measure the flow rate of the gas at the time of a backward flow.
There is known a configuration that limits the inflow of intake air at the time of a backward flow by providing a protrusion on a downstream side of the outlet to improve the accuracy of measurement of the flow measuring device that can measure the flow rate at the time of a backward flow (see, e.g., JP2013-019674A). Specifically, in the flow measuring device in JP2013-019674A, the outlet opens toward the downstream side in the flow direction of the main passage. The measured gas which has flowed out of the outlet at the time of a forward flow flows in the same direction as the flow of the main passage along the flat surface of the outer wall of the housing. The protrusion exists on this flat surface of the outer wall of the housing on a downstream side of the outlet, and this protrusion limits the inflow of intake air at the time of a backward flow.
However, the downstream surface of the shape of the protrusion in JP2013-019674A rises perpendicularly to the flat surface of the outer wall of the housing. Thus, at the time of a backward flow, a flow separation is easily caused near the top of the protrusion, and the amount of air flowing into the sub passage becomes slightly insufficient. Consequently, the measurement accuracy can be reduced depending on the operational state of the internal-combustion engine.